Field of the Invention
The invention relates to an apparatus for controlling and adjusting pulse optimization of a magnetic resonance system and to a method for controlling and adjusting pulse optimization of a magnetic resonance system.
The invention further relates to a magnetic resonance system having a radio-frequency transmitting system, having a gradient system and a control device which is designed to control the radio-frequency transmitting system and the gradient system on the basis of a predefined pulse sequence in order to perform a desired scan, and having a pulse sequence optimization device.
Description of the Prior Art
In a magnetic resonance system, also known as a magnetic resonance tomography system, the body to be examined is usually exposed to a relatively high basic magnetic field, e.g. of 1, 3, 5 or 7 teslas, using a main field magnet system. In addition, a gradient system is used to apply a magnetic field gradient. Radio-frequency excitation signals (RF signals) are then emitted via a radio-frequency transmitting system using suitable antenna devices, which is designed to cause the nuclear spins of particular atoms resonantly excited by this radio-frequency field to be tilted by a particular flip angle relative to the lines of force of the main magnetic field. On relaxation of the nuclear spins, radio-frequency signals, so-called magnetic resonance signals, are emitted which are received using suitable receive antennas and then further processed. The desired image data can then be reconstructed from the thus acquired raw data.
For a particular scan, a particular pulse sequence must therefore be emitted which consists of a string of radio-frequency pulses, in particular excitation pulses and refocusing pulses as well as gradient pulses to be emitted in an appropriately coordinated manner in different spatial directions. Suitably timed readout windows must be set which predefine the time segments in which the induced magnetic resonance signals are acquired. For imaging, the timing within the sequence is critical, i.e. what pulses follow one another at what time spacings. A large number of the control parameters are generally defined in a so-called scan protocol which is created in advance and called up e.g. from a memory for a particular scan and can in some cases be changed locally by the operator who can specify additional control parameters such as e.g. a particular inter-slice separation of a stack of slices to be scanned, a slice thickness, etc. A pulse sequence, which is also termed a scanning sequence, is then calculated on the basis of all these control parameters.
The gradient pulses are defined by their gradient amplitude, the gradient pulse duration and by the rate of change or rather the 1st derivative of the pulse shape dG/dt of the gradient pulses, usually also termed the slew rate. Another important gradient pulse variant is the gradient pulse moment (also called “moment” for short) which is defined by the integral of the gradient amplitude over time.
During a pulse sequence, the magnetic gradient coils via which the gradient pulses are emitted are switched frequently and rapidly. As the timing within a pulse sequence is mostly very strict and, in addition, the total duration of a pulse sequence which determines the total duration of an MRI scan must be minimized as far as possible, gradient strengths of around 40 mT/m and slew rates of up to 200 mT/m/ms are to be reached. In particular, such a high slew rate contributes to the well-known acoustic noise occurring during gradient switching. Eddy currents associated with other components of the magnetic resonance system, in particular the radio-frequency shield, are one reason for these noise problems. In addition, steep edges of the gradients result in a higher energy consumption and also place more exacting requirements on the gradient coils and the other hardware. The rapidly changing gradient fields result in distortions and oscillations in the gradient coils and transmission of these energies to the housing. Heating of the coils and the other components may also cause high helium boil-off.
In order to reduce the noise levels, various hardware design solutions have been proposed, such as e.g. encapsulation or vacuum sealing of the gradient coils.
In DE 10 2013 202 559, corresponding to US 2014/0232396, a method is described in which a prepared pulse sequence, which is to be sent to the scanner of the magnetic resonance system, is analyzed in order to determine a time interval, i.e. a pulse block section within the pulse sequence, which is to be optimized with respect to a gradient waveform, or more specifically a gradient pulse. In a first step, all the original commands that are sent to the scanner are intercepted, then in a second step they are first examined for optimizable segments therein, in a third step these segments are optimized, and not until a fourth step is the optimized pulse sequence forwarded to the scanner. There, the optimization preferably takes place using spline interpolation that satisfies particular constraints including a gradient moment, and in an amplitude at predefined nodes, in particular at a start time of the respective interval and at an end time of the respective interval. Spline interpolation produces a maximally smooth gradient waveform with rounded edges. The calculation of such a spline interpolation is relatively time-consuming particularly in the case of longer gradient intervals, possibly resulting in the scan being aborted if only a small amount of time is available for optimization. This can be problematic particularly for real-time applications.
The first two of the aforementioned steps require only a negligibly small amount of computing time. However, step III (optimization) and step IV (forwarding to the scanner and internal implementation) may involve lengthy calculation times. In the case of real-time applications, or in the case of protocols having very short repetition times, this can result in scans being aborted if the calculation time of the optimization unit and the required implementation time relating to the above mentioned fourth step become greater than a real time need and a small buffer, so the commands cannot be provided at the required real time.
It is possible to reduce the calculation time of the optimized gradient waveforms by ensuring that each waveform is only calculated once, and is reused from then on in each repetition. A linear waveform can also be calculated instead of a fourth order spline. However, the time which is required in the fourth step described above for the provision and internal implementation of the commands does not change as a result of said methods. This calculation time depends on the size of the objects in the transmitted event blocks. Because of the high gradient sampling rates, the gradient commands are critical here for the time duration for the fourth step.